This application claims the priority of Japanese Patent Application No. 128318/1994 filed on May 17, 1994 which is incorporated herein by reference. An optical fiber coupler is a device which is produced by joining a plurality of optical fibers at their middle and has a plurality of inputs and outputs symmetrically and a joined middle part. The coupler distributes a signal from an input to all the outputs. An M:M coupler is made by melting and joining M fibers at the middle. Thus the coupler has M inputs and M outputs reciprocally and a unified part. A signal of an arbitrary input is equally divided into M outputs. A coupler which unifies a plurality of fibers into a single joined part is called a star coupler. The unified part or joined part is produced by bringing the sides of a portion of fibers into contact with each other, heating the contacting portions, melting, and unifying the portions into one. The joined part of the fibers is then stretched. The distances between the cores of two fibers become very short by the stretching. The extreme approach of two cores makes an evanescent coupling between the cores. Thus a signal is exchanged among the fibers. An M:M coupler which has joined M fibers must distribute an arbitrary signal in a fiber to M fibers with an equal power. If a coupler is in trouble, some signal from a sending port is not transmitted to some receiving ports. There has been no system having such a node capable of detecting the troubles of couplers by itself till now. This invention suggests a system which can detect the faults of couplers in the case of the simple 2:2 couplers.